Not applicable.
Clothes washing machines generally contain a tub for holding the clothes being washed. These tubs have a circular bottom and an annular wall extending upward from the bottom""s peripheral edge. They also have numerous drain holes for allowing water to drain freely from the tub during the washing process.
Washing machine tubs are often cast from metal, with the drain holes formed during the casting process. Once cast, these tubs are generally coated with porcelain to provide a smooth surface and a clean appearance.
Washing machine tubs may also be made of molded plastic. Plastic tubs offer advantages over metal tubs in that they are lighter and made from relatively inexpensive starting materials. The manufacture of such tubs, however, may also be time consuming and labor intensive as conventional production methods often require additional steps or further finishing. For example, conventional methods typically require the manufacture to mold the tub in one operation, drill drain holes into the tub in a second operation, and then deburr and recess the drain holes in a third operation.
Recent injection mold assemblies have been configured to avoid the assembly and finishing operations mentioned above. These assemblies are known in the art and often require the use of metal core pins which engage an opposing mold surface to form the drain holes. Unfortunately, these assemblies have also proven to be both expensive to maintain and susceptible to wear. For instance, the core pins on these assemblies often require precise machining within tight tolerances to ensure that each pin is of the correct length to properly match its opposing mold surface. Moreover, the repeated engagement between each core pin and its opposing mold surface will often result in wear on both the mold surface and the pin. This wear, in turn, will typically cause a flashing to form over the molded drain hole, that must be removed before placed into operation. The worn core pin may also require replacement by another core pin machined to the exact length as the replaced core pin.
The removal of the molded tubs from the injection molding machines has also proven to be difficult. This difficulty arises from the shape of the tub and the circular raised solid surfaces on the mold which provide for the recessing of the drain holes. In particular, the raised mold surfaces hinder the removal of the tub as the tub invariable catches on the raised solid surface of the molding machines"" central mold core.
More recent mold assemblies have sought to avoid the above difficulties by altering the design of the drain holes. For example, U.S. Pat. No. 5,881,909 and U.S. Pat. No. 5,287,989 (both incorporated herein by reference) describe an improved injection molded plastic tub having drain holes with teardrop-shaped grooves on the inner surface of the tub sidewall. U.S. Pat. No. 5,167,898 (incorporated herein by reference) also describes a mold assembly and method for producing such a tub. The orientation and configuration of these teardrop-shaped grooves eases separation and removal of the molded tub from the mold core.
Notwithstanding the foregoing, there still remains a need in the art for improved methods of molding plastic washing tubs which facilitate separation of the tub from the mold.
The present invention provides a plastic washing machine tub produced as a one piece, integrally molded and finished unit configured for easy removal from a tub mold assembly. The washing machine tub also includes drain holes arranged to prevent the snagging of fabrics placed in the tub and configured to promote the easy removal of the tub from its mold assembly. The present invention also provides a mold assembly for making such a tub.
One aspect of the present invention is a plastic washing machine tub comprising a substantially circular base wall and a generally cylindrical sidewall extending upward from the peripheral edge of the base wall to a terminal edge. The sidewall has an inner surface and an outer surface and a plurality of drain holes extending therethrough. The inner surface of the sidewall has a raised annulus formed about each drain hole, said annulus being tapered with the height of the raised annulus being greatest proximate to the periphery of the drain hole and gradually decreasing radially outward from the drain hole to form a gradual transition between the raised annulus and the inner surface.
Another aspect of the present invention is an improved injection mold assembly configured to produce the above-described washing machine tub. In one embodiment, the mold assembly comprises a plurality of mold core pins which are engageable with substantially circular depressions on a central mold core to produce the raised annulus on the inner surface of the tub when the mold assembly is closed. The depressions are configured and arranged to facilitate separation of the tub from the central mold core and withdrawal of the tub from the mold assembly. The core pins are preferably elastically deformable in response to engagement with the depressions, reducing the wear on both the core pins and the depressions and, thereby, making the mold assembly more durable. The mold assembly also preferably incorporates selected slanted molding surfaces to provide draft to the sidewalls of the molded tub to further ease removal of the tub from the mold assembly.
Another aspect of the present invention is a method for manufacturing a one-piece, injection molded washing machine tub having a substantially circular base wall and a generally cylindrical sidewall extending upward from the peripheral edge of the base wall, the sidewall including a plurality of drain holes extending therethrough with a tapered, raised annulus formed about each drain hole on the sidewall""s inner surface, said annulus having a height greatest proximate to the periphery of the drain hole and decreasing radially outward therefrom to form a gradual transition between the annulus and the sidewall""s inner surface. The method employs a mold assembly comprising opposed first and second mold dies, a central mold core extending from the first mold die and including a plurality of substantially circular depressions, and a plurality of side mold dies around the mold core, each side mold die having a plurality of core pins. The first step comprises placing the first and second mold dies and the side mold dies in a mold closed position wherein the first and second mold dies and the side mold dies define a mold cavity conforming to the dimensions of the tub. While in this closed position, the core pins engage the substantially circular depressions on the mold core to form drain holes in the tub. Plastic is then injected into the mold cavity to form the molded tub. To remove the tub from the mold assembly, the second mold die is displaced away from the first mold die to an open position relative to the first mold die, and the side mold dies are displaced outwardly to withdraw the core pins from the drain holes formed in the molded tub. The molded tub is then displaced from the mold core.
It is one object of the present invention to provide a method and apparatus for molding a one piece, integrally molded and finished tub that is easy to remove from its mold assembly once molded.
It is another object of the present invention to provide a method and apparatus for molding a one piece, integrally molded and finished tub that does not snag the fabric placed therein.
It is yet another object of the present invention to provide a one-piece, integrally molded and finished washing machine tub which can be easily removed from its mold assembly once formed, and which does not snag the fabrics placed therein.
One advantage of the present invention is that the tapered, raised annuluses disclosed in the present invention facilitate the removal of the molded tub from its mold assembly while preventing the tub""s drain holes from snagging fabrics placed therein.
Another advantage of the present invention is that the tub can be manufactured using resilient core pins and elastically deformable inner pins so as to reduce the wear placed upon the mold assembly while facilitating the use of replacement parts.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings.